There is a transient diffraction grating method as one of methods of measuring diffusion of particles. For example, a method for measuring a diffusion constant by means of the transient diffraction grating method, thereby detecting protein association based upon a change of the diffusion constant has been disclosed (see Japanese Patent Laid-Open Publication No. 2004-85528).
According to the transient diffraction grating method according to the related art, as shown in FIG. 6, two pulse excitation light beams with the same wavelength are radiated onto a sample such that the beams cross each other, thereby forming interference fringes. While molecules (particles) in the sample present in bright portions of the interference fringes formed by the pulse excitation light beams are partially photoexcited, molecules (particles) in the sample present in dark portions of the interference fringes are not photoexcited. Therefore, the photoexcited molecules and the non-photoexcited molecules are alternately present in a regular arrangement in an area where the interference fringes are formed, and a diffraction grating (transient diffraction grating) is temporarily generated in their diffusion process.
When a probe light beam is additionally radiated to the area where the transient diffraction grating is formed, the probe light beam is diffracted by the transient diffraction grating. After the transient diffraction grating is formed of the photoexcited molecules and the non-photoexcited molecules by emitting pulse excitation light, as time goes on, the photoexcited molecules and the non-photoexcited molecules are diffused to be mixed together, so that the transient diffraction grating vanishes. Then, the intensity of the diffracted light beam of the probe light beam generated from the transient diffraction grating decays. The decay curve of the intensity of the diffracted light beam at that time represents a diffusion constant (diffusion coefficient) of the molecules in the sample, and it is thus possible to calculate the diffusion coefficient of the molecules in the sample based upon the decay curve, and to further obtain information on the size (particle diameter), the shape, and interaction with a solvent of each of the particles in the sample based upon the diffusion coefficient.
Particles whose diameters are equal to or less than 100 nm are generally referred to as nanoparticles. Also, nanoparticles have different properties from the bulk materials of the same molecules. For these reasons, nanoparticles have been used in various fields. Various methods of measuring the diameters of particles including a laser diffraction/scattering method have been known. However, in order to measure the diameters of nanoparticles whose diameters are equal to or less than 100 nm, mainly, methods based on a measuring method referred to as a dynamic scattering method (photon correlation method) have been used (for example, U.S. Pat. No. 5,094,532 and Japanese Patent Laid-Open Publication No. 2001-159595).
The dynamic scattering method uses Brownian motion of particles. In the dynamic scattering method, a light beam is radiated to particles undergoing Brownian motion in a medium and the intensity of the light beam scattered by the particles is measured at a predetermined position, so that a fluctuation in the intensity of the scattered light beam variation in the scattered light over time, which is caused by the Brownian motion of the particles, is caught. Also, the distribution of the particles of a particle group to be measured is calculated by using the fact that each particle undergoes the Brownian motion with an intensity based on its diameter.
[Patent Document 1] Japanese Patent Laid-Open Publication No. 2004-85528
[Patent Document 2] U.S. Pat. No. 5,094,532
[Patent Document 3] Japanese Patent Laid-Open Publication No. 2001-159595